1. Field of the Invention
The present invention relates to a method of analyzing a stack of flat objects as well as to a device for analyzing a stack of flat objects. The present invention in particular pertains to a device and a method of analyzing a bundle of banknotes, which method comprises the steps of providing a bundle of banknotes, which bundle comprises at least one surface defined by the edges of banknotes, illuminating the surface of said bundle, providing a two-dimensional image of the bundle by making use of an optical sensor, and providing an output signal that represents the result of the analysis.
2. Description of the Related Art
From International application WO01/50426 there is known a method of determining a characteristic of a banknote including a sheet-like substrate of plastics material and pacifying layers applied to the two outer surfaces of the substrate. The method that is known therefrom comprises die steps of irradiating the substrate, the opacifying layers acting to guide the radiation “within” the substrate, whereupon the emission at the “end” of the substrate is detected, after which one or more characteristics of the emission, such as the intensity or the wavelength, are analyzed. The method described in said International application is only suitable for so-called “polymer banknotes”, because the light bean must be trapped in the substrate.
From U.S. Pat. No. 6,182,962 there is known a method of separating a single note from a stack of banknotes, wherein the thickness of the stack is determined by means of a density sensor. The density is claimed to be a measure of the pressure with which the stack of banknotes is pressed against a withdrawal means. The method that is known therefrom is aimed at removing a single banknote from a stack of banknotes; the stack of banknotes as a whole is not analysed as such, however.
The method referred to in the introduction is also known from U.S. Pat. No. 5,534,690 (corresponding European patent No. 0 805 992). The method of counting stacked banknotes that is known therefrom requires the use of at least one optical sensor, which images at least two separate columns simultaneously along at least one surface of the bundle of banknotes, said columns extending in a direction perpendicular to the surface of the banknotes. On the basis of the signal provided by the optical sensor, a perception of the number of banknotes in the stack is obtained, for example by comparing the two images. One drawback of such a method is the fact that the bundle of banknotes must be subjected to so-called column imaging at two different positions. If the bundle contains folded, torn or strongly creased banknotes, this will render the result inaccurate.
From U.S. Pat. No. 5,918,960 there is known a method wherein a single banknote is illuminated with ultraviolet light of two different wavelengths, wherein detectors are used for detecting reflected light from the banknote having a first wavelength within a first wavelength band and for detecting fluorescence light from the banknote having a second wavelength within a second wavelength band different from said first wavelength band, said second wavelength band including wavelengths at which counterfeit objects may fluoresce when exposed to said ultraviolet light. Such a method is only limited to verifying authenticity characteristics of a single banknote, which means that if a large amount of banknotes is to be verified, each banknote must be separately subjected to such a verification of authenticity characteristics.
Banknotes include authenticity characteristics which may vary with each individual country, region or zone from a few authenticity characteristics in some banknotes to more than twenty authenticity characteristics in the Euro banknotes, for example. Such authenticity characteristics enable the user, the commercial financial institutions and the Central Banks to determine the authenticity of a banknote at different levels. Authenticity verification generally takes place upon acceptance of banknotes. At Central Banks, the verification of the authenticity characteristics of banknotes is carried out by means of so-called banknote sorting machines, with so-called “single note” sorting taking place. This means that all banknotes, which are usually supplied in bundles of 100, 500 or 1000 units, must first be “unbundled”, which is a cost-intensive operation. Subsequently, the unbundled banknotes are mechanically verified one by one, irrespective of their value or their physical condition, by means of so-called sorting machines which carry the banknotes past a series of detectors and sensors. The verification comprises a number of authenticity checks, which can be carried out by means of a machine, as well as all kinds of measurements for determining the present condition or the fitness for use of the banknotes.
Low-denomination banknotes constitute about 40% of the total volume of banknotes that is in circulation worldwide. The “single note” sorting process as described above does not provide a desirable solution for handling low-denomination banknotes, in view of the high sorting costs and the (frequently) poor condition of these banknotes. Moreover, the efficiency of the sorting machine will strongly decrease if the physical condition of the banknotes to be processed is poor. The quality of low-denomination banknotes is generally inferior to that of high-denomination banknotes. This means that the handling costs of lower denomination banknotes are disproportionately high in relation to the value that such banknotes represent. In addition, low-denomination banknotes are rarely counterfeited, so that the high sorting costs will outweigh the security risk.